Maybe change up the lair, for a surprise.
If they are expecting a hot, lava lair I would try for a supernaturally cold one. give the dragon a ring of cold immunity and watch the players freeze.

he can still breath thus melting the ice and dropping the players into whatever is underneath... like acid pools. heck make the "ice" acidic ice. so they take fire damage (probably got resistances) from the breath weapon and then a bit of acid damage from the melted acid (probably not prepped for). and then the ice reforms (or at least begins to reform).

but the real fun begins when the defeated the dragon, with its dieing breath, begins laughing at them, as the moutain/caverns/whatever begining to crack and break ... the dragon was merely the final guardian to stop the mighty TARRASQUE from reawakening!!!!!!!!!!!

Killing most dragons (and most any other creature) is really easy. Just throw some dust of choking and sneezing in their face, then coup de grace them with Dastardly Finish. The ultimate one-two combo.

Can you control the placement of the 20' spread on that? If so, that is horrific..ly awesome! Now how does one go about pricing a cursed item that doesn't normally have a cost associated with it?

Killing most dragons (and most any other creature) is really easy. Just throw some dust of choking and sneezing in their face, then coup de grace them with Dastardly Finish. The ultimate one-two combo.

Can you control the placement of the 20' spread on that? If so, that is horrific..ly awesome! Now how does one go about pricing a cursed item that doesn't normally have a cost associated with it?

Killing most dragons (and most any other creature) is really easy. Just throw some dust of choking and sneezing in their face, then coup de grace them with Dastardly Finish. The ultimate one-two combo.

That MIGHT work, depending on how your DM rules things. Basically, if Dust of Choking & Sneezing is resistant to fire damage/has hardness and hitpoints enough to survive Extreme Heat or even an area filled with fire (unless you catch the dragon outside of her home, but I don't think that was the idea here).

Also, the dragon could simply opt to not try to save against it (at 1200 years and with that intelligence, it will recognize it for what it is), make a quick escape and come back or simply use time stop+limited wish to heal up and get a bit away from them.

It IS a good tactic, no doubt - and even if the dragon chooses to fail the save you'll have a period between the attack and the dragon's round where the dragons Con will be far, far lower than normal so it'll be easier to kill it with damage, but it's no "autowin" button.

EDIT: I'm also thinking that dragons would get some kind of contingency for when people get close to it rather than it getting close to people to protect it from both this and astral rifts. I haven't looked up exactly what type of contingency that would be, but I'm not 1200 years old either so ;D